(1) Field of the Invention
The present invention relates to the field of systems for transmitting flight commands in aircraft, and in particular in rotorcraft, for causing a movable member to be moved in order to modify the attitude in flight of the aircraft. The present invention relates more specifically to drive trains for transmitting flight commands that involve both a trim actuator and a control member for generating manual flight commands.
More particularly, the present invention relates to ways of coupling a said trim actuator and a said control member.
(2) Description of Related Art
Aircraft are fitted with movable members that can be moved by a pilot in order to modify the attitude in flight of the aircraft. By way of example, such movable members are constituted by control surfaces or flaps in an airplane and possibly in a rotorcraft. In the specific situation of a rotorcraft, said movable members are mainly constituted by the blades of at least one rotor of the rotorcraft, said blades being mounted to be pivotable about a pitch axis in order to enable their pitch to be varied cyclically and/or collectively as a function of flight commands issued by a pilot.
Conventionally, said movable members are movable by means of drive trains connecting them to a control member for generating flight commands. Conventionally, such a drive train comprises a mechanical transmission train connecting at least one said movable member to a control member for generating manual flight commands that is movable in tilting by a human pilot.
When automatic flight commands are involved, said drive train also includes an autopilot that generates automatic flight commands for activating the operation of actuators placed in the mechanical transmission train.
For a given mechanical transmission train, the actuators typically comprise two actuators, commonly referred to as a “series” actuator and as a “trim” actuator. The series actuator is placed in series with the mechanical transmission train, being interposed between said control member and the movable member. The trim actuator is placed on the mechanical transmission train in parallel with the control member.
It is nevertheless necessary for the human pilot to know the position of the mechanical transmission train as moved by the trim actuator so that the pilot knows the current position of the movable member independently of it being moved by the control member or by the trim actuator. For this purpose, it is conventional to connect the control member and the trim actuator together mechanically.
The mechanical connection between the trim actuator and the control member is commonly provided via a lever arm fastened to the control member and coupled to a pivot shaft of the trim actuator. The pivot shaft is mounted to be stationary in translation and it is driven by a motor, in particular an electric motor. Operation of the motor depends on commands generated by the autopilot. The motor and the pivot shaft are typically mutually engaged via a step-down mechanism such that rotation of the motor causes the lever arm to pivot and consequently tilts the control member for moving the mechanical transmission train, thereby providing the pilot with knowledge of the current position of the movable member.
Under such conditions, the control member is tiltably mounted on the structure of the aircraft while being mechanically engaged with the mechanical transmission train and being secured to the lever arm that can be tilted by the pivot shaft of the trim actuator under the effect of commands generated by the autopilot.
It can thus be seen that a pivoting movement of the pivot shaft driven by the motor of the trim actuator causes the control member to move in tilting and also causes the movable member to be moved by means of the mechanical transmission train. Conversely, the human pilot imparting a tilting movement to the control member causes not only the movable member to move by means of the mechanical transmission train, but also causes the pivot shaft to pivot.
Furthermore, the trim actuator frequently incorporates one or more clutch mechanisms interposed between the motor and the pivot shaft. The trim actuator may also incorporate a force return mechanism between the motor and the pivot shaft, which mechanism involves elastically deformable members. Such a force return mechanism is commonly used to inform the human pilot via the control member about the position of the movable member relative to a predetermined neutral zone.
It can thus be seen that the trim actuator is a member that is structurally complex. In this context, account needs to be taken of the potential of a malfunction of the trim actuator leading to the pivot shaft becoming blocked at least in part or to it opposing resistance to being moved in pivoting, with the consequence of potentially giving rise to difficulties for the human pilot in moving the control member in tilting.
To solve this problem, it is conventional to place a breakable member, such as a shear pin, in the train for transmitting movement of the trim actuator and connecting together the pivot shaft and the motor.
In the event of a resisting force exceeding a predefined force threshold being opposed to transmitting movement between the trim actuator and the control member, the breakable member is voluntarily broken by the human pilot tilting the control member hard in order to release it from the influence being exerted by the trim actuator. As a result of such arrangements, in the event of the trim actuator malfunctioning, the human pilot can nevertheless move the mechanical transmission train freely in order to move the movable member by using the control member once it has been released of the influence exerted by the trim actuator.
That solution is effective, but uncomfortable for the human pilot. In order to avoid unwanted breakage of the breakable member, said force threshold is defined to be of considerable value. In the event of the human pilot having difficulty in moving the control member because of a malfunction of the trim actuator, the human pilot must then break the breakable member by applying a large tilting force on the control member.
It would be desirable for the influence exerted by the trim actuator on the control member to be released spontaneously in the event of a malfunction giving rise to said resisting force. It would also be desirable for said force threshold to be predefined as accurately as possible. In the event of the control member being released from the influence exerted thereon by the trim actuator, it is also appropriate to ensure that it is not possible for the trim actuator to regain influence spontaneously over the control member in the event of the resisting force dropping below the predefined force threshold.
In this context, document FR 2 931 131 (Eurocopter France) proposes incorporating a torque limiter mechanism in a trim actuator, the mechanism being interposed in the movement train that is incorporated in the trim actuator, interconnecting the pivot shaft and the motor. More particularly, the torque limiter mechanism is interposed between the pivot shaft and an intermediate shaft driven by the motor.
According to FR 2 931 131, the torque limiter mechanism provides rotary coupling between the pivot shaft and the intermediate shaft via a ball abutment. In the event of malfunction of the trim actuator giving rise to a resisting force between the pivot shaft and the intermediate shaft that exceeds a predefined force threshold, the pivot shaft is caused spontaneously to become uncoupled from the intermediate shaft in order to release the control member from the influence exerted by the trim actuator. Such spontaneous uncoupling is made to be irreversible by placing the balls in specific housings of the torque limiter mechanism during the uncoupling operation, with it not being possible for the balls to escape spontaneously from those housings. In order to reestablish coupling between the pivot shaft and the intermediate shaft, human intervention is needed in order to reset the torque limiter mechanism.
Reference may also be made to the following documents, which describe various torque limiter mechanisms between two rotary shafts: EP 1 705 115 (Eurocopter France); DE 10 2012 009149 (Liebherr Aerospace GmbH); U.S. Pat. No. 5,299,666 (D. J. Lang et al.); and U.S. Pat. No. 4,030,578 (M. J. Cacciola, et al.).
Nevertheless, it appears that the structure, the integration, and the modes of operation of such a torque limiter mechanism in the trim actuator could be improved.
It is desirable for the torque limiter mechanism to be integrated in the trim actuator while avoiding as much as possible adding complexity to the structure of the trim actuator. It is also desirable to avoid increasing the weight and the overall size of the trim actuator, and to limit the cost of producing it and of maintaining it.
Furthermore, the organization and the operation of the torque limiter mechanism should enable it to be reset easily so as to reestablish the influence of the trim actuator on the control member. It is essential for such resetting to be performed solely by human intervention, and to ensure that an operator finds it as easy as possible to do, in particular by limiting operations of dismantling the trim actuator. Furthermore, and as mentioned above, the trim actuator potentially includes numerous members that are associated with one another. A malfunction of the trim actuator may thus be caused by a malfunction of any one of these associated members and/or may result from a malfunction of means for causing these associated members to co-operate with one another.
In this context, it is desirable for the release of the control member from the influence exerted by the trim actuator to be provided by means that are located as close as possible to the engagement between the control member and the trim actuator. Nevertheless, it is not appropriate to make the mechanical connection between the trim actuator and the control member via said lever arm any more complex.
The present invention lies in the context of a mechanical transmission train for transmitting flight commands in a rotorcraft in order to move a movable member that modifies the attitude in flight of the aircraft. Such a mechanical transmission train comprises in particular a control member movable by a human pilot in order to generate manual flight commands and in particular also comprises a trim actuator having a pivot shaft driven by a motor and mechanically connected to the control member via a coupling mechanism including a lever arm coupled to the pivot shaft and mechanically connected to the control member. On the basis of the above observation from which the present invention stems, modes of coupling are proposed between the control member and the trim actuator that enable the control member to be released from the influence exerted by the trim actuator in the event of a malfunction. The present invention seeks in particular to provide a coupling mechanism between the control member and the trim actuator that provides as well as possible a satisfactory compromise taking account of all of the above-specified constraints.